Thoughts on Starting the Hydrogen Economy

In their article "The Hydrogen Economy" (PHYSICS TODAY, December 2004, page 39), George Crabtree, Mildred Dresselhaus, and Michelle Buchanan say that "basic research must provide breakthroughs . . . to make a hydrogen-based energy system . . . vibrant and competitive." This statement overlooks the near-term feasibility of an ammonia-mediated hydrogen-based system.1 A research breakthrough might reduce the cost of ammonia production, by emulating its biosynthesis,2 for example. But we have known how to make NH3 economically for almost a century. Nowadays, between 1% and 2% of the world's energy is devoted to synthesizing ammonia from air and hydrocarbons, notably natural gas, via the Haber–Bosch process.3

Because ammonia forms hydrogen bonds, unlike H2 or methane, it liquefies at about 8 atmospheres and room temperature, or ambient pressure and –33 °C. Indeed, because of this favorably situated phase transition, anhydrous ammonia was used as a household refrigerant for much of the 20th century.

Pipelines are in place to distribute anhydrous ammonia. To fertilize their fields, farmers routinely pull tank trucks up to ammonia "filling stations." An ammonia-fueled automobile with an internal-combustion engine was reported in the 1970s.4 Commercial catalytic cells are available to break ammonia into nitrogen and hydrogen and thus produce feedstock for a hydrogen fuel cell. Solid-electrolyte ammonia fuel cells have been demonstrated.5

Because Bosch synthesis is performed in large industrial plants, the carbon dioxide byproduct can be captured and sequestered relatively easily—for example, by pumping it back into the wells that supplied the natural-gas feedstock. Any means of producing hydrogen based on a renewable energy source could substitute for the Haber–Bosch process, and thereby allow for "renewable" ammonia production.

Unlike CH4 and CO2, ammonia is not a greenhouse gas. In the atmosphere, it quickly forms hydrogen bonds to water vapor and returns to the ground in alkaline rain. However, NH3 is toxic, chills its surroundings rapidly on vaporizing, and releases heat on contact with water. Engineering a safe fuel tank for an ammonia-fueled vehicle would be a key priority.

Ammonia is an excellent material for hydrogen storage. As Crabtree and coauthors report in their figure 4, the volume density of hydrogen in liquid NH3 is more than 40% greater than in liquid H2, and the comparison becomes much more favorable when one considers the weight of the required fuel tank and peripherals. Unlike H2 gas, ammonia explodes in air only over a narrow range of concentrations. Shipping ammonia from production site to point-of-use does not require a great deal of cooling or high pressure. Thousands of miles of NH3 pipeline in the US stand as evidence that reliable infrastructure for NH3 transport and storage has been engineered. In sum, liquid NH3 is not just an excellent hydrogen-storage material but also an ideal medium for moving hydrogenic energy from place to place.

Given these advantages, it is hard to avoid the conclusion that relatively modest investments in the science and engineering of NH3 synthesis and fuel cells, and in safer transport, storage, and delivery of NH3, are the best hope for making the hydrogen economy a reality in our lifetimes (and by the way, I am 62).

George Crabtree, Mildred Dresselhaus, and Michelle Buchanan assert that the energy required to split the water molecule and release hydrogen is later recovered during oxidation to produce water. As any undergraduate student of thermodynamics knows, that statement is false; only some of the energy is recovered in any realizable manner. This fact points up the general fallacy in the public's mind about hydrogen being an energy source. Unless and until we are able to connect a hose to Jupiter, hydrogen should be viewed not as an energy source but as a storage medium.

Moreover, as the authors aptly point out, hydrogen does not store energy nearly as efficiently as does gasoline. As long as gasoline is abundantly available, hydrogen will not be cost competitive. Given the stress on the federal budget, large-scale government funding of R&D related to the hydrogen economy is not likely to happen. My guess is that, for the foreseeable future at least, hybrid gasoline technology is where the action will be in the energy sector.

Lewis A. Glenn

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Lawrence Livermore National Laboratory

Livermore, California

As I understand it, if hydrogen is burned, the only "exhaust" is water. We currently send tons of carbon dioxide into the atmosphere; has anyone looked at how much water we'd be sending out, I presume as vapor, under the hydrogen economy and what effect all that water would have? I visualize something like Venus, where the planet's surface ends up covered in a huge cloud—at least as bad as carbon dioxide—that traps in all the heat. Is that what would happen?

Phil Stripling

San Mateo, California

Crabtree, Dresselhaus, and Buchanan reply: Peter Feibelman raises a good point in advocating NH3 as a hydrogen storage medium. He points out many advantages, including its high storage capacity, the significant ammonia infrastructure already in place, and our extensive chemical knowledge and industrial experience with ammonia.

The problem of effective hydrogen storage is one of the most challenging in the hydrogen economy, and we should pursue all promising options. The use of ammonia in a hydrogen economy has been discussed since at least the 1970s; Ali T-Raissi summarizes its history and its possibilities.1 The subject remains vibrant today; new mechanisms for the release of hydrogen from ammonia over catalysts at acceptable temperatures are continuing topics of research.2 A major challenge is toxicity, as Feibelman points out, but all hydrogen storage proposals come with safety issues.

Ammonia can be used effectively in other hydrogen storage media as well,1,3 notably in combination with its borane analog, BH3. NH3BH3 releases more than 12% of its mass as H2 in decomposing to NHBH at low temperature and ambient pressure. Its release rate and decomposition chemistry can be significantly improved by nanoscale structuring in porous hosts.3 This example shows how the richness of hydrogen chemistry and the influence of nano-patterning lead to new horizons in hydrogen storage.

Lewis Glenn correctly points out that the energy used to split water is only partially recovered on recombination of H2 and O2 to make water. No energy conversion process is 100% efficient; some energy will always be lost. The higher potential efficiency of fuel cells over internal combustion engines is an appealing advantage of hydrogen over gasoline. As a carrier of energy, hydrogen costs more to produce than gasoline, whose energy originates naturally in the crude oil from which it is refined. Although gasoline outperforms hydrogen in cost, hydrogen is the winner in the long-term sustainability of supply, security of access, and freedom from environmental pollution and climate change. These long-term quality-of-life issues are strong justification for strategic research now to enable the hydrogen economy in the future.

The switch from fossil fuel to hydrogen replaces emission of the greenhouse gas CO2 with emission of H2O, as Phil Stripling points out. Wouldn't there be a potentially serious environmental impact from that additional water? The hydrogen required to supply the world's energy for one year, 13 TW-yr, would make approximately 31 km3 of water as "exhaust." This is about twice the volume of Crater Lake in Oregon. The total water on Earth amounts to 1.4 × 109 km3, and that in the atmosphere to 12 000 km3. Thus even if all the exhaust water produced in one year from a hydrogen economy remained in the atmosphere, it would increase atmospheric water vapor by less than 1%. The actual increase would be much less, since the residence time of water vapor in the atmosphere is about nine days.